Method and apparatus for monitoring capacitor faults in a capacitor bank

ABSTRACT

A method and an apparatus are disclosed for providing an indication of a capacitor fault in a given string of a capacitor bank comprising at least one string mounted in parallel, each string comprising a plurality of capacitor elements connected in series. The method comprises obtaining a capacitor bank voltage, the obtaining comprising measuring a voltage across the capacitor bank; obtaining a current of the given string, the obtaining of the current of the given string comprising measuring a current flowing in the given string of the capacitor bank; determining a measured impedance at a grid frequency using the obtained capacitor bank voltage and the obtained current of the given string; providing an indication of a capacitor fault if a difference between the measured impedance and a previously measured impedance exceeds a first given threshold for a first given duration.

FIELD

The invention relates to a digital protection relay for protecting powersystems. More precisely, one or more embodiments of the inventionpertain to a method and an apparatus for monitoring capacitor faults ina capacitor bank connected to an AC system.

BACKGROUND

Protecting the three-phase electrical grid and related equipment fromunplanned device short-circuit is very desirable.

Transport and distribution lines of the electrical grid are inductive innature and they connect to a load that is also inductive in nature.Therefore, capacitor banks are required for supplying the reactive powerabsorbed by the connected lines and loads.

These capacitor banks are distributed throughout the electrical networkand operate at various voltage levels, as known to the skilledaddressee.

In many cases, high voltage capacitor banks are formed by connectingmultiple capacitor elements in series to form a capacitor string and byparalleling a certain number of such capacitor strings, thus resultingin a high number of capacitor elements in a given capacitor bank.

Unfortunately, a high number of capacitor elements comes with a higherfailure probability, where a capacitor failure will result in avariation of the capacitor string impedance and, in such an event, in ahigher voltage applied across every other capacitor in the string wherethe failure has occurred.

One prior art method used for determining if a capacitor bank contains afailed capacitor is by measuring the current flowing in the neutralconnection of the three-phase capacitor bank and detecting upon anyimbalance of the bank impedance. Another prior art method is bymeasuring the bank impedance for each of the three phases and generatingan alarm after detecting a variation in the measurement of the bankimpedance of the phase considered. Unfortunately, a limitation of thesetwo methods is the lack of precision with respect to the exact locationof the faults within a capacitor bank and possible early tripping of theprotection circuit breaker thereof, leading to unnecessary down-time.For example, 5 short-circuited capacitor elements could be fatal to thecapacitor bank when occurring in the same capacitor string, whereas 5short-circuited capacitor elements in 5 separate strings can besustained for a long period of time. A more sophisticated and accurateprior art method consists in measuring the impedance of each string ofthe capacitor bank and actuating the circuit breaker in series with thebank only if the variation in one of the string impedances exceeds aprogrammed threshold.

However, a drawback of this prior art method derives from the fact thateach capacitor element may present a natural variation of itscapacitance value with its case temperature, leading to an overallstring impedance variation of a few percent within a 24-hour periodassociated to the effect of the surrounding temperature which may resultin a false alarm to be issued.

There is therefore a need for a method and apparatus that will overcomeat least one of the above-identified drawbacks.

BRIEF SUMMARY

According to a broad aspect, there is disclosed a method for providingan indication of a capacitor fault in a given string of a capacitor bankcomprising at least one string mounted in parallel, each stringcomprising a plurality of capacitor elements connected in series, themethod comprising obtaining a capacitor bank voltage, the obtainingcomprising measuring a voltage across the capacitor bank; obtaining acurrent of the given string, the obtaining of the current of the givenstring comprising measuring a current flowing in the given string of thecapacitor bank; determining a measured impedance at a grid frequencyusing the obtained capacitor bank voltage and the obtained current ofthe given string and providing an indication of a capacitor fault if adifference between the measured impedance and a previously measuredimpedance exceeds a first given fast change threshold for a first givenduration.

In accordance with one or more embodiments, the indication of acapacitor fault is also provided if a difference between a delayedmeasured impedance and a filtered impedance generated by filtering themeasured impedance exceeds a second given threshold for a second givenduration, the filtering comprising performing a temporal filtering forremoving fast variations of the determined impedance over time.

In accordance with one or more embodiments, the indication of acapacitor fault is also provided if a fast variation of impedance isdetected using at least one filter.

In accordance with one or more embodiments, the filtering of themeasured impedance comprises providing the measured impedance if thedifference between the measured impedance and a previously measuredimpedance exceeds the second given threshold for a second givenduration.

In accordance with one or more embodiments, the method further comprisescounting a number of capacitor fault provided for the given string ofthe capacitor bank over a given period of time and storing an indicationof said number in a memory.

In accordance with one or more embodiments, the method further comprisesstoring an indication of the difference between the measured impedanceand a previously measured impedance if the difference between themeasured impedance and a previously measured impedance exceeds a firstgiven threshold for a first given duration.

In accordance with one or more embodiments, the indication of thedifference stored comprises at least one of an amplitude value and apercentage of a nominal value of a capacitor element.

In accordance with one or more embodiments, the method further comprisesadding each of the stored indication of the difference over a given timeduration of interest.

In accordance with one or more embodiments, the method further comprisescomputing a string impedance drift for the given string, the computingcomprising multiplying the number stored in the memory by acorresponding impedance of a capacitor element of the given string ofthe capacitor bank.

In accordance with one or more embodiments, there is disclosed a methodfor identifying a defect in a capacitor bank comprising performing themethod disclosed above for each string of the at least one string of thecapacitor bank.

In accordance with one or more embodiments, the second given thresholdis equal to a proportion of a corresponding nominal impedance of ahealthy capacitor element of the string.

In accordance with one or more embodiments, the fast variations arecharacterized by a frequency greater than ⅕ Hz.

In accordance with a broad aspect, there disclosed an apparatus forproviding an indication of a capacitor fault in a given string of acapacitor bank comprising at least one string mounted in parallel, eachstring comprising a plurality of capacitor elements connected in series,the apparatus comprising a voltage measuring unit operatively connectedto a capacitor bank, the voltage measuring unit for measuring a voltageacross the capacitor bank and providing a signal indicative of thevoltage; a current measuring unit operatively connected to the givenstring, the current measuring unit for measuring a current flowing inthe given string and providing a signal indicative of the currentflowing in the given string; a memory unit and a processing unitoperatively connected to the voltage measuring unit, to the currentmeasuring unit and to the memory unit, the processing unit receiving thesignal indicative of the voltage and the signal indicative of thecurrent flowing in the given string and determining a measured impedanceat a grid frequency using the signal indicative of the voltage and thesignal indicative of the current flowing in the given string, theprocessing unit further generating and providing an indication of acapacitor fault if the processing device determines that a differencebetween the measured impedance and a previously measured impedancestored in the memory unit exceeds a first given threshold for a firstgiven duration.

In accordance with one or more embodiments, the processing unit furtherprovides the indication of a capacitor fault if the processing unitdetermines that a difference between a delayed measured impedance and afiltered impedance generated by filtering the measured impedance exceedsa second given threshold for a second given duration, the filteringcomprising performing a temporal filtering for removing fast variationsof the determined impedance over time.

In accordance with one or more embodiments, the filtering of themeasured impedance comprises providing the measured impedance if thedifference between the measured impedance and a previously measuredimpedance exceeds the second given threshold for a second givenduration.

In accordance with one or more embodiments, the processing unit furtherdetermine a number of capacitor faults provided for the given string ofthe capacitor bank over a given period of time and further stores anindication of said number in the memory unit.

In accordance with one or more embodiments, the processing unit storesan indication of the difference between the measured impedance and apreviously measured impedance if the difference between the measuredimpedance and a previously measured impedance exceeds a first giventhreshold for a first given duration.

In accordance with one or more embodiments, the indication of thedifference stored comprises at least one of an amplitude value and apercentage of a nominal value of a capacitor element.

In accordance with one or more embodiments, the processing unit furtheradd each of the stored indication of the difference over a given timeduration of interest.

In accordance with one or more embodiments, the processing unit furthercomputes a string impedance drift for the given string, the computingcomprising multiplying the number stored in the memory unit by acorresponding impedance of a capacitor element of the given string ofthe capacitor bank.

In accordance with one or more embodiments, the second given thresholdis equal to a proportion of a corresponding nominal impedance of ahealthy capacitor element of the string.

In accordance with one or more embodiments, the apparatus furthercomprises a display unit operatively connected to the processing unit,the display unit for providing the indication of a capacitor fault.

In accordance with one or more embodiments, the apparatus furthercomprises a communication unit operatively connected to the processingunit, the communication unit for providing the indication of a capacitorfault to a remote processing unit operatively connected to thecommunication unit.

In accordance with one or more embodiments, the processing unitgenerates and provides an alarm signal if the number of capacitor faultreaches a given number.

In accordance with one or more embodiments, the indication of acapacitor fault comprises an indication of a defective capacitor.

In accordance with one or more embodiments, the measuring of the currentflowing in the given string of the capacitor bank comprises measuring avoltage at a corresponding capacitive current sensor located in thegiven string.

In accordance with one or more embodiments, the measuring of the voltageis performed via an insulating transformer.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be readily understood, one or moreembodiments of the invention are illustrated by way of example in theaccompanying drawings. In the drawings:

FIGS. 1a and 1b are drawings which illustrate typical connections ofcapacitor banks in an electrical grid. FIG. 1a shows a three-phasenetwork with three capacitors connected between each of the three phasesA, B, C and the common neutral connection. FIG. 1b shows the internallayout of a high-voltage capacitor bank connected to one of the threephases, wherein the capacitor bank contains a number of stringsconnected in parallel, each of which contains a number of capacitorelements connected in series within a given string.

FIG. 2 is a drawing which illustrates a system comprising an embodimentof the apparatus for monitoring capacitor faults in a capacitor bank andincluding string current measuring devices, a bank voltage measuringtransformer and a circuit breaker, connected to a processing unit.

FIG. 3 is a drawing which illustrates an embodiment of the apparatus formonitoring capacitor faults in a capacitor bank. The apparatuscomprises, inter alia, a processing unit, a memory unit, a voltagemeasuring unit and a current measuring unit.

FIG. 4 is a cartesian diagram of a measured impedance of a capacitorstring as a function of time in a typical outdoor application, where theimpedance value varies naturally of a few percent as the temperaturechanges throughout the day. In FIG. 4 is also depicted a short impedancevariation occurring at time 18 hour during a grid transient.

FIG. 5 is a flowchart which shows an embodiment of the method fordetermining if a fault has occurred in a given capacitor string of thecapacitor bank. The method comprises, inter alia, an optional fastchange detection algorithm.

FIG. 6 is a flowchart which shows an embodiment of a fast changedetection algorithm.

FIGS. 7a and 7b are cartesian diagrams depicting an example of theeffect of one or more embodiments of the method disclosed herein in thecase where a voltage transient occurs in the bank voltage. FIG. 7aillustrates the effect of the grid transient on the measured impedance,whereas FIG. 7b illustrates that no capacitor fault is generated upondetecting a grid transient not associated to the failure of a capacitorelement.

FIGS. 8a and 8b are cartesian diagrams depicting an example of theeffect of one or more embodiments of the method disclosed herein in thecase where a capacitor fault occurs in a given string. In FIG. 8a , themeasured impedance is shown as a function of time for a 40-hour timeduration, including a slight reduction in the total string impedanceafter the fault has occurred, whereas FIG. 8b displays the effect of thefault on the output of the low-pass filter, in a preferred embodiment.

FIGS. 9a and 9b are cartesian diagrams depicting an example of theeffect of one or more embodiments of the method disclosed herein, in thecase where one of the capacitor elements, among all capacitors in thestring considered, fails with a short-circuited behavior. FIG. 9aillustrates the effect of the capacitor failure on the measuredimpedance, whereas FIG. 9b illustrates that a capacitor fault isgenerated upon detecting a difference between the measured impedance andthe output of the low-pass filter according to one or more embodimentsof the invention disclosed.

FIGS. 10a and 10b are drawings which illustrate an embodiment of thesystem comprising the apparatus for monitoring capacitor faults in acapacitor bank, in the case where the current flowing in each of thecapacitor strings is sensed by measuring the voltage across a capacitivecurrent sensor. FIG. 10a shows a system configuration using onecapacitive current sensor per string, with the apparatus for monitoringcapacitor faults in a capacitor bank interfacing directly to each of thecapacitive current sensors. FIG. 10b shows an alternate systemconfiguration, where the apparatus interfaces with each of thecapacitive current sensors via isolating transformers.

DETAILED DESCRIPTION

In the following description of the embodiments, references to theaccompanying drawings are by way of illustration of an example by whichone or more of embodiments of the invention may be practiced.

Terms

The term “invention” and the like mean “the one or more inventionsdisclosed in this application,” unless expressly specified otherwise.

The terms “an aspect,” “an embodiment,” “embodiment,” “embodiments,”“the embodiment,” “the embodiments,” “one or more embodiments,” “someembodiments,” “certain embodiments,” “one embodiment,” “anotherembodiment” and the like mean “one or more (but not all) embodiments ofthe disclosed invention(s),” unless expressly specified otherwise.

A reference to “another embodiment” or “another aspect” in describing anembodiment does not imply that the referenced embodiment is mutuallyexclusive with another embodiment (e.g., an embodiment described beforethe referenced embodiment), unless expressly specified otherwise.

The terms “including,” “comprising” and variations thereof mean“including but not limited to,” unless expressly specified otherwise.

The terms “a,” “an” and “the” mean “one or more,” unless expresslyspecified otherwise.

The term “plurality” means “two or more,” unless expressly specifiedotherwise.

The term “herein” means “in the present application, including anythingwhich may be incorporated by reference,” unless expressly specifiedotherwise.

The term “whereby” is used herein only to precede a clause or other setof words that express only the intended result, objective or consequenceof something that is previously and explicitly recited. Thus, when theterm “whereby” is used in a claim, the clause or other words that theterm “whereby” modifies do not establish specific further limitations ofthe claim or otherwise restricts the meaning or scope of the claim.

The term “e.g.” and like terms mean “for example,” and thus do not limitthe terms or phrases they explain. For example, in a sentence “thecomputer sends data (e.g., instructions, a data structure) over theInternet,” the term “e.g.” explains that “instructions” are an exampleof “data” that the computer may send over the Internet, and alsoexplains that “a data structure” is an example of “data” that thecomputer may send over the Internet. However, both “instructions” and “adata structure” are merely examples of “data,” and other things besides“instructions” and “a data structure” can be “data.”

The term “i.e.” and like terms mean “that is,” and thus limit the termsor phrases they explain. For example, in the sentence “the computersends data (i.e., instructions) over the Internet,” the term “i.e.”explains that “instructions” are the “data” that the computer sends overthe Internet.

Neither the Title nor the Abstract is to be taken as limiting in any wayas the scope of the disclosed invention(s). The title of the presentapplication and headings of sections provided in the present applicationare for convenience only and are not to be taken as limiting thedisclosure in any way.

Numerous embodiments are described in the present application and arepresented for illustrative purposes only. The described embodiments arenot, and are not intended to be, limiting in any sense. The presentlydisclosed invention(s) are widely applicable to numerous embodiments, asis readily apparent from the disclosure. One of ordinary skill in theart will recognize that the disclosed one or more embodiments of theinvention(s) may be practiced with various modifications andalterations, such as structural and logical modifications. Althoughparticular features of the disclosed invention(s) may be described withreference to one or more particular embodiments and/or drawings, itshould be understood that such features are not limited to usage in theone or more particular embodiments or drawings with reference to whichthey are described, unless expressly specified otherwise.

With all this in mind, one of more embodiments of the present inventionare directed to a method and an apparatus for monitoring the faults anddefects of capacitor elements in a capacitor bank. It will beappreciated by the skilled addressee that the modern electricalnetworks, hereafter referred to as the electrical grid, contain largecapacitor banks connected to each of the three phases A, B, C of themedium to high voltage transmission and distribution lines. The purposeof these capacitor banks is usually to compensate for the reactive powerconsumed by the distribution and transport grid and by the load. Thecapacitor banks will also contribute to filtering of the grid fromundesirable voltage harmonics. In many cases, these capacitor banks areconnected directly between the medium to high voltage line and theneutral connection of the grid or between two phases of the medium tohigh voltage lines. The result is such that these capacitor banks willneed a string of a high number of discrete capacitors connected inseries, to sustain the grid voltage. As a result, the RMS voltage acrossany of the discrete capacitors will be equal to the RMS line voltagedivided by the number of such capacitors connected in series in thestring.

It is of common engineering knowledge that the series connection ofmultiple capacitors will reduce the overall bank capacitance valuemeasured between the line and the neutral. Therefore, in most cases,there is a need to connect many of these capacitor strings in parallelto increase the line capacitance value. Considering the high number ofseries-connected capacitor in a string and possibly more than one ofsuch strings connected in parallel, the result is a high number ofdiscrete capacitors contained in a capacitor bank.

From an engineering point of view, increasing the number of componentsin any device will usually also increase the potential of failure ordefects in that device. This principle also holds for a capacitor bankconnected to the grid, where a failure of a capacitor element willresult in short-circuiting the failed capacitor element. Two types ofcapacitor banks have been disclosed and are currently implemented:fuseless capacitor banks and fused capacitor banks. In a fuselesscapacitor bank, the failure of a capacitor element will modify theoverall bank capacitance by decreasing its total impedance. In a fusedcapacitor bank, a short-circuit in one of the capacitor elements willresult in blowing a fuse connected in series with the capacitor element,thus resulting in an overall increase in the bank impedance. In eithercase, there will occur a variation (positive or negative) of the bankimpedance and yet, it may still be possible to operate with a failedelement for a certain period of time and obtain acceptable performancewith avoidance of grid down-time.

In all circumstances, it is required that the fault be communicated tothe grid operator, in order to plan for future maintenance. In the casewhere the number of faults would increase beyond a certain level, itwill be required to activate the circuit breaker and de-energize thecapacitor bank. For example, a capacitor string containing 50 capacitorelements may tolerate the defect of one capacitor element, but will nottolerate 10 defects. In the case of a 10-defect scenario, the voltageacross one capacitor element will increase from 100% to 125% of itsnominal value, which would destroy all the capacitors in the string andshort-circuit the grid at that point creating significant damage andloss of power.

Now referring to FIG. 1a , a general schematic of a capacitor bank isillustrated including phase A capacitors 102, phase B capacitors 106 andphase C capacitors 108. In FIG. 1 b, the capacitor bank is shown for onephase only and is formed by a number of strings connected in parallel,each of which contains four capacitor elements in series. A first stringis formed by capacitor elements 141, 142, 143, 144 being connected inseries. The string current 149 flows in each capacitor element and thephase voltage 140 is applied across the entire string.

Now referring to FIG. 2, the method disclosed herein is performed by anapparatus for providing an indication of a capacitor fault 203. Theapparatus for providing an indication of a capacitor fault 203comprises, inter alia, a processing unit and is connected to the currenttransformer 211 to measure the string current 149. The skilled addresseewill appreciate that the current transformers 211, 212, 213 enable amonitoring of the current in each of the capacitor strings in thecapacitor bank, thus enabling the apparatus for providing an indicationof a capacitor fault 203 to measure all the individual string currents149, 159, 189.

FIG. 2 also shows a voltage transformer 214, which enables the apparatusfor providing an indication of a capacitor fault 203 to measure avoltage across the entire capacitor bank. In the embodiment illustratedin FIG. 2, the capacitor bank and the voltage transformer 214 areconnected to the neutral point 220, 221 of the grid. In an alternativeembodiment, the capacitor bank is connected between two phases and theconnection points 220, 221 are to be replaced with a phase connection.

It will be appreciated by the skilled addressee that the measurement ofthe voltage of the bank and each of the current of the individualstrings enables the apparatus for providing an indication of a capacitorfault 203 to determine an impedance value for each of the strings.

According to one or more embodiments of the method disclosed herein, arapid change of the impedance of a capacitor string is detected by theprocessing unit of the apparatus for providing an indication of acapacitor fault 203. The embodiment presented in FIG. 2 also discloses adisplay unit 202 which is used for visually informing a grid operator ofany possible failure or default detected in any of the given strings. Itwill be appreciated that a circuit breaker 204 is usually connected inseries with the capacitor bank, which in a preferred embodiment will betriggered by the apparatus for providing an indication of a capacitorfault 203 upon detecting a change in the string impedance that exceeds agiven trip threshold.

Now referring to FIG. 3, there is disclosed a preferred embodiment ofthe apparatus for providing an indication of a capacitor fault 203. Theapparatus for providing an indication of a capacitor fault 203 comprisesa processing unit 301, a voltage measuring unit 310, a current measuringunit comprising a first Analog-to-Digital Converter ADC 311, a secondADC 312 and a k^(th) ADC 315, where k refers to the number of capacitorstrings in the capacitor bank. The apparatus for providing an indicationof a capacitor fault 203 further comprises a display unit interface 304and an input/output interface 303.

It will be appreciated that the processing unit 301 may be of varioustypes. In one embodiment, the processing unit 301 comprises amicroprocessor. In an alternative embodiment, the processing unit 301comprises a digital signal processor (DSP). In another alternativeembodiment, the processing unit 301 comprises afield-programmable-gate-array (FPGA). The skilled addressee willappreciate that various alternative embodiments may be possible for theprocessing unit 301.

The voltage measuring unit 310 is used for measuring a voltage of thecapacitor bank. It will be appreciated by the skilled addressee that thevoltage measuring unit 310 may be of various types. In one embodiment,the voltage measuring unit 310 comprises an analog to digital converter(ADC) 310. The skilled addressee will appreciate that variousalternative embodiments may be possible for the analog to digitalconverter (ADC) 310.

The current measuring unit is used for measuring a current in eachstring of the capacity bank. It will be appreciated by the skilledaddressee that the current measuring unit may be of various types. Inone embodiment, the current measuring unit comprises a first ADC 311, asecond ADC 312 and a k^(th) ADC 315. The skilled addressee willappreciate that various alternative embodiments may be possible for thecurrent measuring unit, which may interface to a set of k currenttransformers in one embodiment. In an alternative embodiment, the k ADC311, 312, 315 comprised in the current measuring unit may also interfaceto a set of k capacitive current sensors.

The apparatus for providing an indication of a capacitor fault 203further comprises a memory unit 302 operatively coupled to theprocessing unit 301. In one embodiment, the memory unit 302 is used forstoring data, such as for instance fault status. The skilled addresseewill appreciate that various types of memory units may be used.

The apparatus for providing an indication of a capacitor fault 203further comprises a display interface 304. The display interface 304 isoperatively connected to the processing unit 301 and is used foroperatively connecting the apparatus for providing an indication of acapacitor fault with a display unit, not shown. The skilled addresseewill appreciate that various embodiments may be possible for the displayinterface 304 depending on the display unit used. It will be appreciatedthat the indication of a capacitor fault may comprise an indication of adefective capacitor.

Still referring to FIG. 3, it will be appreciated that the apparatus forproviding an indication of a capacitor fault 203 is also connected to acircuit breaker interface 320 which enables the triggering of anactuation signal to the circuit breaker 204, if required.

It will be appreciated that the processing unit 301 is used fordetermining a measured impedance at a grid frequency using the signalindicative of the voltage and the signal indicative of the currentflowing in a given string. The processing unit 301 further generates andprovides an indication of a capacitor fault if the processing devicedetermines that a difference between the measured impedance and apreviously measured impedance stored in the memory unit 302 exceeds afirst given threshold for a period of time exceeding a first givenduration.

In one embodiment, the processing unit 301 further provides anindication of a capacitor fault in the case where the processing unit301 determines that a difference between a delayed measured impedanceand a filtered impedance generated by filtering the measured impedanceexceeds a second given threshold for a second given duration. It will beappreciated that the filtering removes fast variations of the measuredimpedance over time as further explained below. It will be furtherappreciated that in such embodiment, the filtering of the measuredimpedance may comprise outputting the value of the string impedance ifthe difference between the measured impedance and a previously measuredimpedance exceeds the second given threshold for a period of time longerthan a second given duration.

In one embodiment, the processing unit 301 further determines a numberof capacitor faults for the given string of the capacitor bank over agiven period of time and further stores this number in the memory unit302. In one embodiment, the processing unit 301 stores an indication ofthe difference between the measured impedance and a previously measuredimpedance if the difference between the measured impedance and apreviously measured impedance exceeds a first given threshold for afirst given duration.

It will be appreciated that in one embodiment, the indication of thedifference stored comprises at least one of an amplitude value and apercentage of a nominal value of a capacitor element.

It will be appreciated that the processing unit 301 further sums each ofthe stored indication of the difference over a given time duration ofinterest in accordance with one embodiment.

In another embodiment, the processing unit 301 further computes a stringimpedance drift for the given string and the computing comprisingmultiplying the number stored in the memory unit 302 by a correspondingimpedance of a capacitor element of the given string of the capacitorbank. In one embodiment, the second given threshold is equal to aproportion of a corresponding nominal impedance of a healthy capacitorelement of the string.

In accordance with one embodiment, a communication unit, not shown, isoperatively connected to the processing unit 301, the communication unitis used for providing the indication of a capacitor fault to a remoteprocessing unit, also not shown, operatively connected to thecommunication unit. In accordance with one embodiment, the processingunit 301 generates and provides an alarm signal if the number ofcapacitor fault reaches a given number.

Now referring to FIG. 4, there is illustrated an advantage of one ormore embodiments of the method disclosed herein. In fact, it will beappreciated that one or more of the embodiments of the method disclosedherein will not interpret as a failure of a capacitor element a naturalvariation of the string impedance which occurs in normal operation ofthe capacitor bank. In fact, the skilled addressee will appreciate thatcapacitor banks are typically placed in outdoor environment, withpossible temperature variations ranging from 20 to 40 degrees Celsius ina 24-hour period, and possibly extreme temperature span of more than 50degrees Celsius in the course of a one-year period. Such largetemperature variations will affect the physical behavior of thedielectrics inside the capacitor elements in a way that the capacitancevalue will change with temperature. A change in the capacitance of eachcapacitor element in a string will result in a natural variation in thetotal string impedance, as illustrated in FIG. 4, where the stringimpedance naturally varies between a lower value 403 and a higher value405 in the course of a same day.

It will be appreciated that such a natural occurrence of a stringimpedance variation with the temperature must not trigger a failurealarm. The method disclosed herein advantageously enables a triggeringof a failure alarm upon a rapid change in the measured impedance,whereas a slow variation of the string impedance is not be interpretedas a failure of the capacitor string.

Now referring to FIG. 5, there is shown an embodiment of a method formonitoring a change in the string impedance and distinguishing acapacitor defect from a natural variation of the bank capacitance due totemperature.

According to processing step 501, a capacitor bank voltage is measured.

According to processing step 502, a current of the given string ismeasured.

According to processing step 503, a measured string impedance Z_m at thegrid frequency is determined. It will be appreciated that the measuredstring impedance is determined using the measured capacitor bank voltageand the measured current of the given string.

According to processing step 504, a status indicating if a fast changeis detected is provided along with the sum Z_cumul_error of the valuesof all fast changes detected so far.

It will be appreciated that processing step 504 comprises a Fast Changealgorithm, which comprises processing steps 601 to 622, detailed hereinbelow and in FIG. 6.

Now, being understood that the processing step 504 provides a status ofwhether a fast change in the measured impedance is detected, a test isperformed, according to processing step 505, in order to determine if aFast Change is detected.

According to processing step 506, a low-pass filter filtering, alsoreferred to as a slow filter, is performed. It will be appreciated thatthe filtering has, in one embodiment, a cut-off frequency lower than 0.2Hz and is implemented digitally in accordance with one embodiment.Electronic analog filtering may also be comprised within processing step506 in accordance with another embodiment. More precisely, the measuredimpedance Z_m is filtered and a slow frequency component of Z_m isoutputted provided that no fast change has been detected by the FastChange algorithm performed at processing step 504. In the case where theFast Change algorithm performed at processing step 504 provides a Statussuch that a Fast Change has occurred, the filtering performed atprocessing step 506 will adjust its output so that its output is equalto the latest value of Z_m provided according to processing step 503. Itwill be appreciated that such a reset of the slow filter used atprocessing step 506, as soon as a fast change is detected, will enable afast reaction of the system where multiple capacitor failures couldoccur in a short time span which is of great advantage.

According to processing step 507, the output of the slow filterdescribed in processing step 506 is used as an estimate of thetemperature-dependent impedance of the capacitor string considered,wherein processing step 507 will yield Z_slow.

According to processing step 510, a value Z_comparison is calculated bysumming Z_slow and Z_cumul_error respectively outputted at processingstep 507 and at processing step 504. It will be appreciated that theZ_cumul_error output of processing step 504 will be adjustedsimultaneously with the output of the slow filter function performed atprocessing step 506. This feature ensures that Z_comparison calculatedat processing step 510 will maintain its value before and after adetection of a fast change.

According to processing step 511, Z_thresh is established as aproportion of Z_comparison.

It will be appreciated by the skilled addressee that processing steps504 to 511 are optional in the embodiment disclosed FIG. 5. The skilledaddressee will therefore appreciate that the use of processing steps 504to 511 discloses an alternative embodiment of the method disclosedherein.

According to processing step 520, a test is performed in order to findout if the condition Z_m−Z_comparison>Z_thresh is true. It will beappreciated that a capacitor defect is detected during this processingstep. The impedance Z_m that was determined at processing step 503 iscompared to the value Z_comparison. If the difference between theimpedance Z_m and the value Z_comparison is small compared to areference threshold value Z_thresh, a capacitor fault status is notissued, whereas larger difference between Z_m and Z_comparison for atime period longer than t_oper will be interpreted as a faulty capacitorelement in the string.

According to processing step 521, a String Fault Status flag is set to 0if the condition tested at processing step 520 is not true. In otherwords, if the measured impedance Z_m remains approximately equal to thecomparison value Z_comparison, no fault is detected.

According to processing step 522, a String Fault Status flag is set to 1if the condition tested at processing step 520 is true. In other words,if the measured impedance Z_m varies from the comparison valueZ_comparison by a value exceeding Z_thresh, a fault is detected.

The fast change detection algorithm performed at processing step 504comprises the detection of a fast variation of the measured impedanceZ_m and resets the slow filter output at processing step 506 such thatZ_slow=Z_m in order to be quickly ready for upcoming fault events. Thefast change algorithm performed at processing step 504 also cumulatesthe total variation Z_cumul_error in the string impedance caused bydefects in the capacitor elements. Z_cumul_error is updated with theamplitude of the fast variation at the same time as the slow filteroutput is reset to the value Z_m. Thus, it ensures that the comparisonvalue Z_comparison stays the same just before and just after a fastchange is detected. It will be appreciated that this processing step isof great advantage as it enables the slow filter to continue to feedthrough the slow component of the measured impedance after a fault eventhas been detected. If the total variation Z_cumul_error was not added tothe comparison value Z_comparison, the fault would clear on its own asZ_slow would slowly converge to the new value of the value Z_m at ratedictated by the time constant of the slow filter.

Now referring to FIG. 6, there is disclosed an embodiment of the FastChange Algorithm. The Fast Change Algorithm is performed at processingstep 504 shown in FIG. 5 and described previously.

According to processing step 601, the last M samples of the measuredimpedance value Z_m are stored. In one embodiment, the last M samples ofthe measured impedance value Z_m are stored in an impedance buffer.

According to processing step 602, a difference Z_diff is calculatedbetween the impedance of the last sample of the measured impedance andthe first sample of the measured impedance which is the first storedmeasured impedance in the impedance buffer, giving a non-zero value uponany change in the measured impedance within the last M samples. In oneembodiment, the difference is calculated using a comb filter. Theskilled addressee will appreciate that various alternative embodimentsmay be used for calculating the difference.

According to processing step 603, the last value of Z_diff extracted atprocessing step 602 as well as the M−1 previous values of Z_diffextracted at processing step 602 are stored in a difference bufferZ_diff_buffer. The Z_diff buffer therefore comprises M elements.

According to processing step 604, a test is performed in order to findout if the difference buffer Z diff buffer contains at least T elementswith values greater than a threshold value Z_event_thresh. If thecondition Z_diff>Z_event_thresh is true for T samples, a rapid change ofthe measured impedance is detected.

The skilled addressee will appreciate that checking the condition for anumber of T samples out of the M elements contained in the Z_diff bufferwill enable a rejection of false positive detection caused for instanceby noise or very short transients. On the other hand, increasing thevalue of T will increase the response time of the rapid changedetection.

In the case where the absolute value of Z_diff exceeds Z_event_threshfor at least T samples, as determined at processing step 604, acumulative error impedance Z_cumul_error is updated by increasing itsvalue to take into account the faults previously recorded and the newfault detected in the capacitor string. It will be appreciated that theindication of the difference stored, i.e. the cumulative error impedanceZ_cumul_error comprises at least one of an amplitude value and apercentage of a nominal value of a capacitor element.

According to steps 620 and 621, the impedance buffer is used forestimating respectively the impedance before the fast change eventyielding Zb and after the fast change event yielding Za. It will beappreciated that the impedance buffer of processing step 601 contains Msamples, a portion of which corresponds to impedance measurement samplesbefore the fast change event, another portion corresponds to impedancemeasurement samples after the fast change event.

According to processing step 622, the sum of the current fast eventZb−Za is provided to the cumulative error impedance Z_cumul_erroraccumulator. It will be appreciated that the cumulative error impedanceZ_cumul_error will contain the sum of the current detected fast changeamplitude and all the previous fast changes amplitudes.

According to processing step 610, the value of the cumulative errorimpedance Z_cumul_error is stored in the memory unit 302.

Now referring to FIGS. 7a and 7b , it will be appreciated that themethod disclosed herein does not generate a fault status in cases wherethe measured impedance undergoes a sudden variation centered around agiven impedance value as depicted in FIG. 7a . This case was disclosedin FIG. 4, where a rapid burst 404 occurs due to a voltage transient inthe phase voltage. Thanks to the action of the slow low-pass filter, theminute-range impedance value does not change. This is shown in themeasured impedance Z_m holding the same value of 3900 ohms before andafter the burst 404. As a net result, the Fault Status 720 displayed inFIG. 7b maintains a low state 721, which is of great advantage.

Referring now to FIGS. 8a and 8b , a short-circuit 804 in the capacitorstring considered occurs at time t=22 hour. FIG. 8b shows the output ofthe slow minute-range low-pass filter, where the instantaneousdisturbance 804 is not recorded, but where the long term filtered valueof the measured impedance is modified after the short-circuit of thecapacitor element has occurred. The curve 810 displays the behavior ofthe measured capacitance in the case with no defect occurring in theconsidered string, whereas curve 811 shows the filtered measuredimpedance after such a short-circuit has occurred.

The skilled addressee will appreciate the difficulty of illustrating theeffect of the method disclosed herein on a time scale of 5 hour per timedivision. For a better understanding of the various mechanisms impliedby the method disclosed herein, FIGS. 9a and 9b provide a detailedanalysis of the different variables for a time scale of 0.1 second perdivision. In FIG. 9a is shown the disturbance 804 associated to theshort-circuit of one of the capacitors in the string. The event occursat the dotted line 903, after which the measured string impedance Z_mdecreases by a value of 60 ohms over a time period of about 0.1 second,after which the measured impedance keeps a steady-state value of 3840ohms. The skilled addressee will appreciate that the slow minute-rangelow-pass filter will retain a value of 3900 ohms during a periodextending over the graph window, due to the large time constantassociated to the filter. This drift between the output of the slowlow-pass filter and the fast variation of the measured string impedanceis one of the main constituents of method disclosed. The skilledaddressee will easily appreciate that the two variables 901 and 906 canbe subtracted from one another, thus obtaining a difference that is tobe compared to a threshold impedance. If the condition ofZ_m−Z_comparison>Z_thresh remains true for an operating time, hereindefined as the time difference between the dotted line 922 and thedotted line 903, then a fault status of 1 is issued as 924 indicated inFIG. 9b after the time line 922.

Now referring to FIG. 10a , it will be appreciated that the methoddisclosed herein is performed by an apparatus for providing anindication of a capacitor fault 203, where in an alternative embodiment,the apparatus 203 is connected to capacitive current sensors 191, 192,193 and processes the voltage measured across each of these capacitivecurrent sensors to derive the value of the current 149, 159, 189 flowingin each of the strings of the capacitor bank. This alternativeembodiment of the system may be implemented in the cases where the gridoperator will prefer using such capacitive current sensors instead ofthe more common current transformers. As a matter of fact, using a veryaccurate value for such capacitive current sensors will lead to anaccurate determination of the currents 149, 159, 189 which will beproportional to the voltage across the capacitive sensors and alsoproportional to the capacitance value of the capacitive current sensorsused.

It will be appreciated by the skilled addressee that connecting theapparatus 203 to the capacitive current sensors 191, 192, 193 throughisolating transformers 196, 197, 198, as shown in FIG. 10b enables ahigher degree of safety and protection against any hazardous gridmalfunction or overvoltage. Such isolating voltage transformers 196,197, 198 may also contribute to stepping down of the measured voltagefor a better adaption to the low voltages needed at the input of theapparatus 203.

It will be appreciated that there is also disclosed a method foridentifying a defect in a capacitor bank wherein the method disclosedabove is performed for each string of the at least one string of thecapacitor bank.

Although the above description relates to a specific preferredembodiment as presently contemplated by the inventor, it will beunderstood that the invention in its broad aspect includes functionalequivalents of the elements described herein.

1. A method for providing an indication of a capacitor fault in a givenstring of a capacitor bank comprising at least one string mounted inparallel, each string comprising a plurality of capacitor elementsconnected in series, the method comprising: obtaining a capacitor bankvoltage, the obtaining comprising measuring a voltage across thecapacitor bank; obtaining a current of the given string, the obtainingof the current of the given string comprising measuring a currentflowing in the given string of the capacitor bank; determining ameasured impedance at a grid frequency using the obtained capacitor bankvoltage and the obtained current of the given string; providing anindication of a capacitor fault if a difference between the measuredimpedance and a previously measured impedance exceeds a first giventhreshold for a first given duration.
 2. The method as claimed in claim1, wherein the indication of a capacitor fault is also provided if adifference between a delayed measured impedance and a filtered impedancegenerated by filtering the measured impedance exceeds a second giventhreshold for a second given duration, the filtering comprisingperforming a temporal filtering for removing fast variations of thedetermined impedance over time.
 3. The method as claimed in claim 1,wherein the indication of a capacitor fault is also provided if a fastvariation of impedance is detected using at least one filter.
 4. Themethod as claimed in claim 2, wherein the filtering of the measuredimpedance comprises providing the measured impedance if the differencebetween the measured impedance and a previously measured impedanceexceeds the second given threshold for a second given duration.
 5. Themethod as claimed in claim 1, further comprising counting a number ofcapacitor fault provided for the given string of the capacitor bank overa given period of time and storing an indication of said number in amemory.
 6. The method as claimed in claim 1, further comprising storingan indication of the difference between the measured impedance and apreviously measured impedance if the difference between the measuredimpedance and a previously measured impedance exceeds a first giventhreshold for a first given duration.
 7. The method as claimed in claim6, wherein the indication of the difference stored comprises at leastone of an amplitude value and a percentage of a nominal value of acapacitor element.
 8. The method as claimed in claim 6, furthercomprising adding each of the stored indication of the difference over agiven time duration of interest.
 9. The method as claimed in claim 5,further comprising computing a string impedance drift for the givenstring, the computing comprising multiplying the number stored in thememory by a corresponding impedance of a capacitor element of the givenstring of the capacitor bank.
 10. A method for identifying a defect in acapacitor bank comprising performing the method as claimed in any one ofclaim 5 and claim 6 for each string of the at least one string of thecapacitor bank.
 11. The method as claimed in claim 2, wherein the secondgiven threshold is equal to a proportion of a corresponding nominalimpedance of a healthy capacitor element of the string.
 12. The methodas claimed in claim in claim 2, wherein the fast variations arecharacterized by a frequency greater than ⅕ Hz.
 13. An apparatus forproviding an indication of a capacitor fault in a given string of acapacitor bank comprising at least one string mounted in parallel, eachstring comprising a plurality of capacitor elements connected in series,the apparatus comprising: a voltage measuring unit operatively connectedto a capacitor bank, the voltage measuring unit for measuring a voltageacross the capacitor bank and providing a signal indicative of thevoltage; a current measuring unit operatively connected to the givenstring, the current measuring unit for measuring a current flowing inthe given string and providing a signal indicative of the currentflowing in the given string; a memory unit; and a processing unitoperatively connected to the voltage measuring unit, to the currentmeasuring unit and to the memory unit, the processing unit receiving thesignal indicative of the voltage and the signal indicative of thecurrent flowing in the given string and determining a measured impedanceat a grid frequency using the signal indicative of the voltage and thesignal indicative of the current flowing in the given string, theprocessing unit further generating and providing an indication of acapacitor fault if the processing device determines that a differencebetween the measured impedance and a previously measured impedancestored in the memory unit exceeds a first given threshold for a firstgiven duration.
 14. The apparatus as claimed in claim 13, wherein theprocessing unit further provides the indication of a capacitor fault ifthe processing unit determines that a difference between a delayedmeasured impedance and a filtered impedance generated by filtering themeasured impedance exceeds a second given threshold for a second givenduration, the filtering comprising performing a temporal filtering forremoving fast variations of the determined impedance over time.
 15. Theapparatus as claimed in claim 14, wherein the filtering of the measuredimpedance comprises providing the measured impedance if the differencebetween the measured impedance and a previously measured impedanceexceeds the second given threshold for a second given duration.
 16. Theapparatus as claimed in claim 13, wherein the processing unit furtherdetermine a number of capacitor fault provided for the given string ofthe capacitor bank over a given period of time and further stores anindication of said number in the memory unit.
 17. The apparatus asclaimed in claim 13, wherein the processing unit stores an indication ofthe difference between the measured impedance and a previously measuredimpedance if the difference between the measured impedance and apreviously measured impedance exceeds a first given threshold for afirst given duration.
 18. The apparatus as claimed in claim 17, whereinthe indication of the difference stored comprises at least one of anamplitude value and a percentage of a nominal value of a capacitorelement.
 19. The apparatus as claimed in claim 17, wherein theprocessing unit further add each of the stored indication of thedifference over a given time duration of interest.
 20. The apparatus asclaimed in claim 16, wherein the processing unit further computes astring impedance drift for the given string, the computing comprisingmultiplying the number stored in the memory unit by a correspondingimpedance of a capacitor element of the given string of the capacitorbank.
 21. The apparatus as claimed in claim 14, wherein the second giventhreshold is equal to a proportion of a corresponding nominal impedanceof a healthy capacitor element of the string.
 22. The apparatus asclaimed in claim 13, further comprising a display unit operativelyconnected to the processing unit, the display unit for providing theindication of a capacitor fault.
 23. The apparatus as claimed in claim13, further comprising a communication unit operatively connected to theprocessing unit, the communication unit for providing the indication ofa capacitor fault to a remote processing unit operatively connected tothe communication unit.
 24. The apparatus as claimed in claim 16,wherein the processing unit generates and provides an alarm signal ifthe number of capacitor fault reaches a given number.
 25. The method asclaimed in claim 1, wherein the indication of a capacitor faultcomprises an indication of a defective capacitor.
 26. The method asclaimed in claim 1, wherein the measuring of the current flowing in thegiven string of the capacitor bank comprises measuring a voltage at acorresponding capacitive current sensor located in the given string. 27.The method as claimed in claim 26, wherein the measuring of the voltageis performed via an insulating transformer.